The concept of fetal pain is becoming increasingly relevant due to growing possibilities for invasive intrauterine treatment. There is much debate as to whether the fetus is mature enough to be able to perceive pain at all. Recent studies have suggested that the fetus is at least capable of exhibiting a stress response to intrauterine needling. Intrauterine transfusions are most commonly performed by inserting a needle either in the umbilical cord root at the placental surface, or in the intrahepatic portion of the umbilical vein of the fetus. Recently, intrauterine needling in the intrahepatic vein has been shown to result in alterations in fetal stress hormones, which has been interpreted as a reaction to pain. These changes were not observed in intrauterine needling in the umbilical cord root, or after administration of analgesics to the fetus. The investigators tested the hypothesis that remifentanil provides fetal analgesia, assessed by a reduced fetal stress response. The investigators performed a randomised controlled trial comparing fetal stress response between patients undergoing intrauterine transfusions for alloimmune fetal anemia receiving remifentanil, or placebo.